1. Field of the invention
This invention relates to a boring device, and more particularly to a drilling machine with an improved efficiency for removing chips formed by a boring tool.
2. Description of Related Art
A boring tool such as an annular cutter or a twist drill which has cutting edges at its tip is formed with flutes in its outer peripheral wall which are continuous with the cutting edges. Chips formed during the boring operation are taken out of the hole along the flutes. Chips are broken at such a length as is determined by the material of a workpiece, a cutting speed and a feed speed of the tool, and other factors.
When a thrust is applied to the boring tool at a substantially constant feed speed, the chips are relatively long. As the chips are continuously formed, their weight and removal resistance increase, lowering the chip removal efficiency of the tool. Thus, chips are clogged in the spaces between the flutes and a hole being bored. The increased cutting resistance impairs the free cutting performance, reducing the boring efficiency, and the cutting edges are undesirably worn out. Further, friction heat makes the cutting edges soft.
In order to solve the above problems, there has been proposed a chip breaking member fixed to a drill casing disposed above the cutting tool such that the breaking member hits against a continuous chip to forcibly break the same due to impact or resistance given by the breaking member.
When such a chip breaking member is disposed adjacent to the boring tool, however, it hinders the boring operation. In addition, the continuous chip may take a helical shape or meander. Thus, it is not assured that the breaking member hits against the chip. The chips may be continuously removed without being broken.
Further, the usage of a gun drill has been proposed. The gun drill supplies cutting oil under a high pressure through the boring tool to chips being removed so that the cutting oil causes the chips to flow out of the spaces between the flutes and the hole. However, this requires a special tool or a tool head. Therefore, this proposal is not suitable for a boring device such as a portable drill press or a relatively small drilling machine.
U.S. Pat. Nos. 2,458,929, 2,514,758, 2,514,759 and 2,474,720 disclose drill presses each provided with a chip breaker which performs chip breaking during the return movement of the spindle. However, each of these chip breakers breaks the chip into pieces having one set length, and thus lack the flexibility to break the chip into a plurality of lengths, a given length being chosen in accordance with the prevailing boring conditions.